Systems for producing transparency images by passing penetrative radiation through a subject and impinging it on an image producing element, such as a photographic film or paper, generally employ one or more radiation absorbing grid assemblies interposed between the radiation source and of the film. The grid assemblies reduce clouding of the photographic image which otherwise results from the effects of scattered radiation. Because of the radiation absorbing property of the grid assemblies, images interposing grid assemblies between the radiation source and the image producing elements can result in a grid image being formed on the image producing elements. Various steps have been taken to minimize such grid images.
The construction of most grid assemblies is accomplished using thin strips of radiation absorbent material, such as lead, oriented edgewise with respect to the radiation source. Radiation sources such as X-ray tubes are, for practical purposes, point sources of the radiation. Therefore the grid assemblies are constructed so that each grid strip extends edgewise and radially with respect to the radiation source focal spot when the grid assembly is in its exposure position between the radiation source and the image producing element. Aligning the grid strips edgewise to the direction of radiation from the focal spot substantially reduces the extent of the grid images produced during exposures, but requires that the grid assemblies be accurately located at predetermined exposure positions. Significant misalignment creates undesired grid strip images and severely reduces the quantity of radiation passing through the grid assembly.
The prior art has also suggested moving the grid assemblies during the time an exposure is made to effectively "blur" the grid image which might otherwise be produced. Because of the grid strip alignment with the radiation source the grid assemblies, when moved during an exposure, cannot be moved far from their exposure positions without the grid strips and source becoming excessively misaligned. Grid assembly motion has therefore commonly been of a cyclic nature with the grid assemblies moved relatively short distances.
The prior art has proposed various schemes for moving grid assemblies during exposure. In some proposals grid assemblies have been supported, at least in part, by resiliently flexible springs which are loaded and released just prior to an exposure being made. The grid assembly is oscillated by the springs whose oscillations are naturally dampened by friction and thus remain pronounced during the period of the exposure.
One problem with the use of grid assembly oscillating springs has been that the grid assembly motion is generally sinusoidal in nature. That is to say, the grid assembly velocities tend to be quite low during a relatively appreciable interval when the grid assembly is in the vicinity of its limits of travel. This results in the grid assembly being imaged when stopped or moving slowly.
If radiation exposure times are relatively brief, there is a chance the exposure could occur when the grid assembly is nearly stationary at an end of its oscillatory travel and a grid image can be produced. If exposure times are in excess of one or two seconds, the grid assemblies repeatedly slow and stop at substantially the same locations tend to "build up" composite grid images.
An alternative approach, suggested by the disclosure of U.S. Pat. No. 2,767,323 issued to Stava et al. on Oct. 16, 1956, employs a motor driven cam for oscillating the grid assembly. The cam drives the grid at relatively low frequency oscillations, but the cam profile is configured so that the grid assembly velocity remained relatively constant up to the point at which the grid assembly changes directions. The grid assembly motion is thus not sinusoidal.
Furthermore, the Stava patent suggests the inclusion of a switching arrangement by which a short, high intensity exposure is prevented while the grid assembly is momentarily stopped at its limit of travel. For long duration exposures the switching arrangement is not used because of the low oscillation frequency.
The construction of these grid assembly oscillating arrangements has been such that the grid assembly oscillation frequencies and grid assembly velocities have had to be relatively low in order to provide adequate grid motion with small oscillation amplitudes. This has been particularly true of spring supported grid assemblies, although the cam driven grid approaches have also depended on relatively low frequency operation so that grid assembly motion is assured throughout the major extent of most exposures without discontinuing the exposure at the limits of grid assembly travel.